I see a LOT of changes being made to the story to allow for this to be a movie. The biggest will be that Master Chief will NOT be wearing a helmet, or at least he won't be for the majority of the film. In a video game world you can relate to your characters by playing through their perspective. In a movie it will be too hard for the audience to relate to the emotions and ordeals of a character when they can't see their face.
Besides, you know that this movie will have a big name star in it, and why would a studio pay a big name to hide under a bunch of armor and a helmet? I'm also eagerly awaiting the love scene between the Chief and hot-ass-hologram-babe-o-the-year Cortana.
